Reposition nested FancyCaptcha, conditional help.

There's less work to do here when the Refresh link is already nicely
positioned over the FancyCaptcha image (in extension/ConfirmEdit's
I57be0bc8aa , merged and deployed).  It still needs separate divs, one
with a border exactly 270px wide (display: table-cell) and one to act as
the positioning parent of the Refresh link (display: inline-block).

Also, only show the help msg for the captcha if it exists.

Bug: 47699
Change-Id: I003c5f1b75471cce32b149037198c314ce9489ea
Merge "Reposition nested FancyCaptcha, conditional help."